Actuators(Take class notes)Actuator types• There are four kinds of actuators commonly used in robotic systems:- Electrical DC motors (operated by electricity)- Hydraulic (operated by fluid)- Pneumatic (operated by air)- Electric stepped motor• We will focus on the DC motor since it is the most commonly usedAdvantages and disadvantages (1) 1. Precision:- Electrical and hydraulic are very precise if controlled properly- Pneumatics work with a compliant “fluid” which makes precise control in continuous-path difficult- Stepper motors are very precise but can lose steps if acceleration/deceleration is not controlled (it is used as an open-loop device)2. Power:- Hydraulic drives generate significant power in a compact volume- Stepper motors have much less torque/size than DC motor- Rare-earth magnets in DC motor generate strong magnetic fieldAdvantages and disadvantages (2)Advantages and disadvantages (3) 3. Cost:- High precision electrohydraulic valves are very expensive, more than electric amplifiers - Stepper motors – for small torques they are rather cost-effective- Pneumatic – very inexpensive4. Summary- DC motors are most commonly used even in heavy-duty applications- Hydraulic systems are used for high power requirementPrinciple of the Electric DC motors (1)• Robots use mostly DC permanent magnet motor• DC permanent magnet motor characteristics- Work on the principle that a wire carrying a current in a magnetic field experiences a forcewhere ∅is the magnetic flux which is due to a permanent magnet, Bthe magnetic flux density (Webers/m2) ∅ iil ∅BSouthNorthffrAxis of rotation• Motion of the armature due to the generated torque is opposed by the inertia of the armature rotor, the voltage generated by the moving coil in the magnetic field, and the load on the motor shaft due to friction, damping, or work done• Voltage generated by the moving coil in the magnetic field is called back electromotive force (back emf)• Speed of rotationBack emfVelocity:Length of wirePrinciple of the Electric DC motors (2)Model of motorR: resistance of armature winding; < 1 L: inductance of armature winding – lowVb: back emfm: motor torque = Ki which is proportional to the currentLm: torque delivered to loadn: speed reducer ratioL: torque received by the load = nLmBm: friction dampingJm:inertial loadRLVbimBmJm ,, , ,LnLmfV++-- /nRobot jointDC motor dynamic equation+ ++ Frictional dampingInertial loadStatic frictionTorque delivered to
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